Wood usage

The wood store is relatively impressive – or so people have been kind enough to say. Essentially it’s the space between the two sheds with a corrugated plastic roof over it and a slatted back so that the air can move through it. Over the spring and early part of the summer all the driest wood I’d collected, from whatever source, got cut up and/or split and stacked in this space so that for the second half of the summer it got a final drying. There were 55 builder’s barrow-loads. The quality of the wood is extremely variable. There’s some really good oak and ash there. Plus some very worm-eaten pine – which had spent quite a lot of time on the ground before being dealt with and had been very well attacked by some wood-boring grub which made deathwatch beetle look positively benign. And lumber right down to a couple of hardboard-on-cardboard doors. These take space and burn well – but not for long!

The logburner in use

There were two evenings in April when we had to light the log burner. Here it is shown alight. The pile of logs to the left is about 3 barrow-loads and the black thing on the wood is a glove which came with the stove. The only bit of the stove which gets really hot is the door on the front – the top and sides are fine to touch.

The Autumn article covers the generation and use of electricity. It could be argued that the Solar PV is nothing to do with having an low energy house. More important in purist terms is how the space heating is performing.

Below is the chart showing when we lit the log burner since mid year until the end of last week and how many barrow-loads of logs have been used in the same period.

Log burner use and consumption 2013

We’ve used 21 barrow-loads of wood in this period. This, as mentioned elsewhere, has provided us with all the hot water we need (for showers, washing up, etc. – the usual DHW – and we certainly do not belong to the cold shower fraternity!) and has kept the house warm as well. It’s been good to note that there was no need to light the fire on either 19th or 28th December. These were lovely sunny days and warmed both the house and the thermal store to meet all our needs for that day and night.

And, while we use the wood saved over the past few years and the lumber left over from building the house itself (there are quite a few pallets to be got through yet) it has cost nothing but my effort and some electricity to drive the saws. The builders, Touchwood Homes, calculate that, if we had had to buy wood in it would have cost £371 based on a cost of 4.2p/kWh. I’ve never bought firewood, so I can’t validate this.

We’ll keep on counting though the rest of the winter and spring so that there’s a full year’s worth of data on wood usage.

14 Responses to Wood usage

Hi Guys – very interesting blog, thank you. We are planning a similar build and just can’t live without a woodburner. Did you need to have an airbrick for your woodburner? And do you find it’s cold to the touch when not in use, due to the thermal bridge through the roof? Plus , does the brick surround retain heat until morning. We are not sure if we might have an inset stove built into a brick block, for slow release heat. Thanks.

Hi SusiQ, Well, yes, I suppose you would need an air-brick because the fire needs air. But, bearing in mind that the idea is that it’s an air tight house (ours is anyway, as I’ve been banging on about) I’d bring the air to the stove in an entirely separate duct so that it’s room-sealed and neither puts fumes into the house nor draws air out of it. Ours starts about a metre up outside, with a grid to deter insects, and then comes underground and up through the floor slab in the right place. There will be some heat loss through the roof but with some clever work by Dragon Contracts, who did the work on this, I don’t think it’s large. And when the stove is not in use, you shut the dampers so that air is not drawn through the stove and flue. The brick surround gets slightly warm, which isn’t a problem, but is not used as a high temperature heat store, only as part of the house’s thermal mass. I don’t have any experience of using an inset stove, but I doubt you would be able to store anything like the amount of heat in a controllable manner in this way that you could in a thermal store. Only the front glass of the stove gets really hot because it is surrounded by its water jacket on the sides and back – it puts most of its output into water and hence into the thermal store. The brick is there for decorative purposes (part of the room’s focus; second-hand soft reds and lime mortar) and is also part of the house’s thermal mass. Regards, David

Hi SuziQ. Yes, it could. Indeed, it does, although we’ve never used it. I was persuaded to have it installed because a) it’s much less expensive to do so while the house is being built and b) because it was mentioned that future purchasers might be wary of a house that didn’t have “recognised” heating system. But we’ve found that the duct heaters are perfectly satisfactory. And as I know that the UFH pipework is very near the bottom of the slab – 150 mm concrete above the pipes – the response time is going to be measured in hours probably many. The UFH has its own programmable thermostat, is partitioned into 4 zones, 3 of which are along the north of the house, with the 4th under the downstairs ensuite – and are fed from the bottom half of the thermal store, same as the duct heaters are.
There’s another consideration. The thermal capacity of the UFH – or, rather, of that part of the slab near it – is large. We’re lighting the log burner for few hours – 2, 3 sometimes 4. To heat the slab I think we’d have to burn it for much longer because we’d be putting much more heat “into store”, at least initially. One day we’ll have to try it.

Hi SuziQ: the two duct heater are made by VEAB Heat Tech SB – and I know nothing about them. They were installed by Touchwood Homes as part of the ventilation system. Here’s a link I have just found http://www.veab.com/en/duct-water-heat-cool/produkter/kanal-varme-kyla-vatten. As explained earlier, water from the lower half of the thermal store is pumped through them when the relevant thermostat calls for heat.
So, no, we are not talking about an ASHP. I couldn’t justify even a GSHP here – there is mains gas available – let alone an ASHP. The recent reports on the efficiency of heat pumps does not make good reading. Prices are, of course, changing all the time, but the last time I worked it all out, electricity is 3 times the price of (mains) gas, and oil is about 1.4 times the price of gas. So if the CoP of an ASHP is 2, oil central heating is less expensive. From the carbon emissions point of view, electricity might be better depending on what was used to generate it!

SuziQ, the duct heaters are heated by the water in the lower half of the thermal store. So, strictly, it’s not (just) the log burner that heats them – it depends what has heated the water in the thermal store. At this time of year it is more and more becoming the sun and the heat stays in the thermal store until late in the evening or perhaps in the morning when one of the thermostats calls for heat an hour or two before we get up!

Hi SuziQ, Sorry for the slow reply. To be fair, I didn’t – Dragon Contracts, the thermal engineers did. The size of the thermal store needs to be matched to the heat demand of the house in terms of both the DHW requirement and the heating requirement. And the number of solar thermal panels needs to be matched to the size of the thermal store. Here, with 2 people in the house, 1,000L seems to be OK, but we would be lighting the log burner more often if we had 4 or 5 people living in the house, I think, as we’d need to have the water in it hotter – unless it was larger. A larger store enables you to carry over more than a day – by which I mean that a good day’s sunshine will put enough heat into the store to last for 2-4 days whereas a smaller store won’t hold enough heat for that long. Also you can do some rough calculation. Ignore volumes of water below 40C for space heating and below 50C for DHW. So if you know how much hot water your household uses per day you can calculate how much energy this needs (to heat water from cold at, say, 10 C to usable at 50C) and so how much heat this will draw from the thermal store. This link http://www.yourhome.gov.au/energy/hot-water-service won’t give you the answers but sets out the principles quite well.

Hi,
We’re planning our own self-build and I’m really interested in using solar thermal, complemented by a wood-burner to heat water and feed UFH.
I understand that if we have a well-insulated relatively air-tight house, we’ll need quite a small stove, with its own direct vent and a damper (to shut off when not in use).
Can you tell me what model of stove you have?
Also, how did you work out the capacity you need for the thermal store?
Thanks

Hi Elaine,
Yes, it makes sense – I’d say essential – to use a room sealed stove, or you pull cold draughts into the house. And it removes the possibility of CO fumes getting into the house. We don’t have a damper as such, just the primary and secondary air controller levers which we shut when the fire’s not in use so that potentially cold air does not flow through the fire and flues, cooling the house! The stove is a Woodfire F12. Reason for choice? At the time it was not possible to buy a good looking pellet stove which put its heat into water – they were all space heaters and that would over-heat the house. Those that heated water clearly belonged in a boiler house. (And we had quite a lot of wood.) That’s no longer true – the market has moved on. Downside – you have to be there to feed the stove with wood. But we quickly found that we never have to run it for more than 4 hours a day. See the reply to SuziQ’s question re the size of the thermal store. We’ve found that 8m2 (net) of thermal panel (south facing 40 deg roof) feeding 1,000L of thermal store in a 297m2 house with 2 people works well. the Resol controller is an important part of the system, helping to make best use of whatever sunshine is available.

Hi, I am interested in the woodfire f12 stove you have installed. Do you have any further comments now you have used it for a while? good points/bad points about its operation? Would you recommend it?
Many thanks,
Tom

Sorry to take so long to reply – I’ve not been monitoring this for a while, partly because the whole thing was being transferred to a new domain and WordPress provider.
The Woodfire F12 is fine. The version that we ended up getting had been recently (=early 2012) updated because, I was told, there had been complaints that the air-wash didn’t work at all well and the glass got smoked up. This sounds as though it’s better. The air-wash is not, in my view, well balanced. The secondary air comes in at both sides and at the bottom of the door and, once the fire is alight, that’s all you use. The primary air comes through the holes in the bottom of the firebox and, as I say, once it’s going you turn that off. But the draught from the left is better than from the right so that, if logs are at all close to the glass on that side, it will soot up. From the point of view of being room-sealed it’s not perfect. When we were carrying out the air tests on the house, we found we had some leaks. For day to day use we’re very pleased with it – the criticisms above are minor.
Regards, David